timestamps: Remove TIMESTAMP_CACHE_IN_BSS

This was implemented for LATE_CBMEM_INIT support which
has already been deprecated.

Change-Id: I39225ba675bc3389e051e15b400a905431969715
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35375
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/timestamp.h

@@ -28,8 +28,9 @@
  */
 void timestamp_init(uint64_t base);
 /*
- * Add a new timestamp. Depending on cbmem is available or not, this timestamp
- * will be stored to cbmem / timestamp cache.
+ * Add a new timestamp. For ENV_ROMSTAGE_OR_BEFORE, this timestamp will be stored
+ * inside REGION(timestamp) before cbmem comes online. For later stages, timestamps
+ * added before cbmem_[recovery|initialize] calls will be lost.
  */
 void timestamp_add(enum timestamp_id id, uint64_t ts_time);
 /* Calls timestamp_add with current timestamp. */